docs: add summary documentation on credentials types and improve YARD comments

[viacheslav-rostovtsev]

This PR adds a principal getter to credentials. The principal is safe to log and is implemented as follows:

1. APIKeyCredentials - returns :api_key
2. BearerTokenCredentials - returns :bearer_token
3. GCECredentials - returns :gce_metadata
4. IAMCredentials - returns the selector
5. ServiceAccountCredentials - returns the service account email (issuer)
6. ServiceAccountJwtHeaderCredentials - returns the service account email (issuer)
7. ImpersonatedServiceAccountCredentials - returns the source principal when available, or :unknown
8. UserRefreshCredentials - returns client_id or falls back to :user_refresh
9. UserAuthorizer - returns the client ID
10. ExternalAccount::BaseCredentials - returns the audience

Also added a Credentials.md documenting the available credentials.

There are also minor doc and comment improvements along the way.
This is to be used in the error details work, to reduce the size of upcoming PR.

[dazuma]

Make sure you update the conventional commit tag before merging.